This invention relates to the detection of an abnormal temperature within the heat transfer element of a regenerative heat exchanger and particularly relates to such a system for rotary regenerative air preheaters.
It has long been known that gas to air regenerative heat exchangers can sometimes experience an excessively high temperature which may lead to a fire within the confines of the heat transfer surfaces. The heat containing gases are typically the exhaust flue gases from a combustion process. As these hot flue gases are directed through a rotary regenerative heat exchanger, the fly ash and unburned products of combustion carried by the flue gas are deposited on the surface of the heat exchanger plates. These deposits continue to build up until air and flue gas flow through the heat exchanger are reduced at least in the region of the deposits. This causes the temperature to rise to the point where the deposits glow and cause a hot spot. If not detected and corrected, this can lead to fires in the heat exchanger. Early detection of such hot spots and fires is critical to containment and correction.
The typical air preheater is normally run at steady-state conditions, with the gas and air inlet temperatures and the gas and air flow rates being nearly constant over a long period of time. However, at one time or another, every air preheater goes through some kind of transient, due to a change in either the air or gas inlet temperatures or in the air or gas flow rates or some combination of these. For example, when an air preheater supplies combustion air to a boiler, the air preheater experiences a transient when the boiler is going through a start-up, a shut-down, or a change in load.
Superimposed on the fluid temperatures measured during steady-state or transient conditions are normal and continuous (but small) stochastic fluctuations. It is also possible that even under steady-state conditions, there may be differences in the air outlet temperature from one rotor compartment to another. These compartment-to-compartment variations in temperature could be due to non-uniform fouling, corrosion, or plugging of the heat transfer matrix in the various compartments of the rotor.
In order to detect an abnormal temperature condition within the heat transfer matrix without setting off a spurious alarm signal, it is necessary for the temperature monitoring system to be able to differentiate between the normal temperature changes caused by transients, stochastic fluctuations, and rotor non-uniformities, and an abnormally high temperature caused by a fire. This means that the relative magnitudes of the various normal fluctuations must be estimated or measured, so that the alarm set point can be defined to be at some level above the worst-case normal fluctuation. In most instances, the greatest fluctuations will be caused by transients due to changes in the gas or air inlet temperatures. Stochastic fluctuations should be quite small, probably on the order of 1.degree. F. Fluctuations due to rotor non-uniformities will vary from unit to unit, but their magnitude would probably lie somewhere between 1.degree. and 10.degree. F. Fluctuations due to transient operating conditions could be greater than 10.degree. F.
There have been many systems proposed and used for the detection of these hot spots. Typically, prior art hot spot detectors have relied on a single pass of the hot spot past a sensor which detects the hot spot on the basis of a predetermined fixed temperature threshold above which a measured temperature is considered to be abnormal. The use of such a fixed threshold temperature ignores the fact that a number of normal variables, as mentioned above, could cause the temperature to exceed the fixed threshold and trigger an alarm or initiate corrective action in unwarranted situations. Examples of such systems include infrared detectors which measure the temperature of the rotor. Infrared detection systems are relatively expensive and require significant maintenance of the detectors and electronics. Prior art thermocouple systems have used the fixed set point method with its attendant disadvantages.